acoustics and laser bathymetry

7/27/2019 Acoustics and Laser Bathymetry

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ACOUSTIC AND LASER BATHYMETRY SYSTEMS

SABUJ DAS GUPTA, ADAM ZIELINSKI

Department of Electrical and Computer Engineering, University of Victoria,3800 Finnerty Road, Saanich, BC, V8N 1M5, Canada,

[email protected]

This paper discusses acoustic and lasers based bathymetric systems in terms of their applicability in diverse circumstances. The advantages and disadvantages of each method arecompared in terms of capabilities, cost and accuracy. None of these systems can provide full bottom coverage in all circumstances but could be supplementary to each other. Informationhas been presented in support of this conclusion. Acoustic bathymetry is suitable at deeper waters whereas laser bathymetry may be used in shallow clear coastal waters. A hybrid option has been suggested with the mix of these systems for higher survey efficiency and lesser costs. This paper is of interest to persons involved in ocean acoustics study and survey

projects planners as well as to the developers of laser instruments for study ocean water and bottom properties and object detection such as wracks, boulders and other objects.

INTRODUCTION

Ocean bathymetry is used primary to produce precise marine navigation charts needed particularly in shallow water. Other applications include dredging operations, oil and gas

exploration, underwater constructions and bottom object detections like boulders, wracks etc.Acoustic multibeam swath bathymetry (MSB) is widely used in the developed countries. Thesystem generates hundreds of narrow beams over which sound pulses are transmitted towardsto and reflected from the bottom to cover a wide swath during a single transect. Bathymetricsurveying is an expensive and labour intensive task. This is one of the reasons for the largehydrographical backlogs being experienced by many countries [1].

Airborne Lidar Bathymetry (ALB) is the other possible approach that could offer quick and accurate survey for both large and small areas, at lower cost but applicable only toshallow, clear waters. The development of ALB from the 1970s parallels the development of MSB. Early systems were rather slow and ponderous but advancement in underlyingtechnologies has made both ALB and MSB much more effective.
mailto:[email protected]:[email protected]


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ALB has the ability to perform survey where it would be difficult, dangerous or impossible to use water-borne techniques. It is capable of measuring the water depth from 20cm down to 60 m, depending on the water clarity [5].

Fig.2. Sample waveform for ALB [2].

It provides the capability to map environmentally sensitive or dangerous coastal areassuch as the areas offshore from the rain forests of Puerto Rico to sensitive sandgrass dunes tothe rocky coasts of Hawaii or New Zealand efficiently and with minimum disruption of theenvironment [6]. It was also possible to survey with minimal impact on fauna and flora in

extremely shallow waters (depths less than a meter) in Florida Bay. It has also improved theability to map various terrestrial maps in a multitude of environments [9, 10].

2. MULTIBEAM SWATH BATHYMETRY

Acoustic MSB is complex but convenient method for surveying the sea bottom. It hasrevolutionized geological mapping [12, 13] with the wide swath advantage and wassuccessfully used to map benthic habitats [14]. This technology effectively covers the seafloor with acoustic pulses that provide an array of measured depths. It uses a transducer totransmit a fan shaped array of pulsed energy divided into a series of narrow receiving beams.The return signal is associated with an individual transmit-receive beam. This makes each

beam unique and derives a separate sounding and measurement of the seafloor reflection.Modern multibeam systems might have up to 800 beams varying from 0.5 degrees to

2.0 degrees in width. Additional configurations and variations among systems can lead todouble pings, resulting in twice the number of soundings per beam. Various transmitfrequencies are utilized by different multibeam sonar systems depending on the sea depth. For example, low frequency (12 kHz) systems can operate swath soundings at full ocean depths,many up to 11,000 meters. In contrast, high frequency systems (300+ kHz) are utilized for swath bathymetry in depths of less than 50 meters. For a sample see Tab.1.

The swath footprint of the sonar ping on the seafloor increases with the depth. It isgenerally one tenth of the water depth. There are many instances however, when bathymetricdata misrepresents the actual seabed topography, due to survey and processing errors and

biases. In [16] Miller described in details the adverse effects of various survey parameters onacoustic seabed coverage.


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Tab.1. Specifications of Multibeam Bathymetry SEA BEAM Systems.

Shallow water Medium water Deep waterModel 1185 1180 1055/1050 1055D 1050D 3050 3030 3020 3012ParametersSoundingrate(Pulses/s) 180 180 50/50 50/180 50/180 50 30 20 12

Number of beams 126 126 126/126 126/126 126/126 918 918 301 301Max m swathcoveragesector 153 153 153 /153 153 /153

153 /153 140 140 140 140

Max m depth(m) 300 600 1500/3000

1500/1500

3000/3000 3000 7000 8000 11000

Along-ship beamwidth 1.5 1.5 1.5 /1.5 1.5 /1.5

1,1.5 or 3 /1,1.5 or 3

1, 1.5or 3

1, 1.5or 3

1 or 2 1 or 2

Across-ship beamwidth 1.5 1.5 1.5 /1.5 1.5 /1.5

1 or 2 /1 or 2

1 or 2

1 or 2

1 or 2 1 or 2

Pulse length(ms) 0.15 -3 0.15 -3 0.15 10/

0.15 10

0.3, 1.3,10/0.15,0.3,1.3

0.3, 1.3,10/0.15,0.3, 1.3

0.15 -10

0.40 -10

3 - 20 3 - 20

Weight (kg) 183 183 365/365 110/110 110/110 750 750 >1000 >3000

3. ADVANTAGES OF AIRBORNE LASER BATHYMETRY

Airborne Laser Bathymetry is designed for surveying the ocean depth but can also beused to detect rocks, sunken ships or men made objects under the water. Unlike the MSBsystems, ALB can be used only for shallow waters but it can be used safely for areas withrocks or other objects which threaten the vessel [17]. It is light that allows operation fromsmall aircrafts. Some systems have demonstrated capacity to collect both hydrographic andterrestrial measurements in a single survey [15].

Accuracy and ability to detect objects. ALB has proven that its accuracy meets theInternational Hydrographic Organization (IHO) standards [7] of order-1 or greater (see Tab.2). Specifically, underwater objects were detected within less than half a meter from their actual locations. As a bathymetric tool ALB is capable of measuring water depth from 0.2 mdown to 60m, depending on the water clarity [18]. The results obtained from laser and sonar surveys have been compared by different experiments. According to [19], 95% of data arewithin 0.24 meters for a LADS Mk II benchmark of 84,500 points for depths ranging from 6-30 meters. This significantly exceeds IHO Order-1 vertical accuracy requirement of 0.50 m.Positive reports were also available from the SHOALS experiment in the United Statescoastal areas [20, 21]. Subsequently, SHOALS results were compared with an operational

National Oceanic and Atmospheric Administration (NOAA), National Ocean Service sonar survey in Tampa Bay up to 20-m depths [22]. The accuracy of SHOALS in that test wasdetermined to be 0.28 meters at the 95% confidence level and greatly exceeds IHO Order-1requirements. IHO standards


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Tab.2. Minimum Standards for Hydrographic Surveys [7].

Order Special 1a 1b 2

Description of areas For areaswhereunder-keelclearance iscritical andrequires fullsea floor search.

Areas shallower than 100 m whereunder-keelclearance is lesscritical butfeatures of concern tosurface shippingmay exist andrequires full seafloor search.

Areas shallower than 100 m whereunder-keelclearance is notconsidered to bean issue for thetype of surfaceshippingexpected totransit the areaand does notrequire full seafloor search.

Areasgenerallydeeper than100 m wherea generaldescription of the sea floor is consideredadequate anddoes notrequire fullsea floor search.

Horizontal accuracy (m) 2 5 + 5% of depth 5 + 5% of depth 20 + 10% of depth

Depth accuracy for reduced depths (m)

a = 0.25 b = 0.0075

a = 0.5 b = 0.013

a = 0.5 b = 0.013

a = 1.0 b = 0.023

Feature detection Cubicfeatures >1 m

Cubic features >2 m, in depths upto 40 m; 10% of depth beyond 40m.

Not Applicable NotApplicable

Recommended maximum

line spacing

Not defined

as full seafloor searchis required.

Not defined as

full sea floor search is required

3 x average depth

or 25 m,whichever isgreater. For

bathymetric lidar a spot spacing of 5x5 m.

4 x average

depth.

Positioning of fixed aids tonavigation and topographysignificant to navigation(m) 2 2 2 5Positioning of the coastlineand topography lesssignificant to navigation(m) 10 20 20 20Mean position of floatingaids to navigation (m) 10 10 10 20

Tests of the Japan Coast Guard SHOALS-1000 also have demonstrated horizontal andvertical accuracy well within IHO standards [22]. FugroPelagos [23] indicated LIDAR depthsmatching the multibeam control results to within IHO accuracy requirements at the 98%confidence level for normal bottoms and 93-94% for bottom with wrecks.

Cost. Cost is affected by area size, distance from the base station, resolution andaccuracy required, environment etc. Flat, smooth, nearby coastlines such as the Gulf Coast of the U.S. are cheaper to survey than the remote, rocky coastlines of Alaska by a factor of two

or three. The average cost of $123/km 2 for ALB and $772/km 2 for MSB, was estimated in[24]. According to [25] study, contractor-operated ALB system predicted a cost benefit ratio


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(a) (b)Fig.3. Survey coverage using ALB (yellow) and boat- Ability of ALB to obtain coverage in wetted-

mounted acoustic depth sounder (blue) side channels, Yakima River, Washington [2].

Tab.4 presents various technical parameters of three modern ALB systems (HawkEye II,

SHOALS-1000 and SHOALS-1000T).

Tab.4. Specifications of ALB systems.

SHOALS-1000 HawkEye-II SHOALS-1000TSounding rate (pulses/s) 1000 4000 1000Sounding footprintdiameter (m) 2,3,4,5 1.7 to 3.5 2,3,4,5Minimum depth (m) 0.2 0.3 Not specified.Maximum depth (m) 50 70 65Aircraft altitude (m) 200-400 250-550 200-400Swath width Up to 0.58 altitude

Typical (215m at 4mDiameter)

100-330 m 60-230 m

Weight 210 kg 190 kg 162 kgSystem power requirements 60 A at 28 VDC 50 A at 28VDC 50 A at 28VDCDepth accuracy IHO order 1 (25cm,

1) IHO order 1 or better (0.25m)

IHO order 1 or better

Horizontal accuracy IHO order 1 (2.5cm,1)

IHO order 1 or better (2.5m)

IHO order 1 or better

Camera 16001200 Pixel s 16001200 Pixel s 16001200 Pixel s

4. LIMITATIONS OF AIRBORNE LASER BATHYMETRY

Green and infrared lasers are used in ALB system. The amount of energy required totransmit a green laser pulse is much higher than that of an infrared laser pulse. Due to the


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requirement of eye safe condition the spot size at surface must be sufficiently large (more than1.7 m in diameter) to reduce power intensity. However, this will decrease accuracy andresolution of the system.

Surface foam caused by ocean waves can make water penetration difficult. In thesecases the productivity of ALB is greatly reduced [32, 33]. Its spatial resolution is not as goodas for modern high frequency MSB. For these reason small targets may not be detected evenif illuminated [34]. The effective approach for increasing the detection probability for smalltarget is to increase the survey density. However, it should be noted that it is also difficult for MSB to detect one-meter cubes on the bottom [35]. A major limitation for ALB systems issurface and water clarity, bottom reflectivity and solar background. These limit the maximum

penetration depths. Other environmental factors, such as rain, high winds, high waves, surf zone, fog, very steep slopes etc. also cause problems with ALB survey [36]. In the rainy daysALB surveys do not take place because backscatter by the raindrops. It is also preferred not tosurvey through fog and clouds. In order to reflect the off-nadir laser energy from the water surface back to the receiver, the capillary waves are needed for non-specular laser reflectionand for this reason a certain low winds are needed. However, the modern ALB systems canovercome this issue, as they use Raman scattering from infrared laser.

5. ADVANTAGES OF ACOUSTIC BATHYMETRY

Depth sounders are affected by platform motion. Modern multibeam systems however use stabilized beams and heave compensation. Measuring the movement and position of thevessel is imperative to preserve the accuracy of the data because the position of the soundingwill change as the vessel moves in the water due to roll, heave, pitch and yaw. Horizontal andvertical positioning is precisely measured through the use of GPS and inertial measurement

units (IMU). Sophisticated IMU is used to compensate the errors caused by the vesselmovements. MSB can map swaths of the bottom in the time it takes for the echo to returnfrom the farthest angle. The SEA BEAM 2100, a multibeam sonar system generates up to 151

beams at 1 apart with each ping, and can cover areas tens of kilometers wide in depths of afew kilometers [37]. It also collects backscatter information, which can provide informationabout the seafloor. Backscatter is characterized by the intensity or strength of the returnedsignal. It is a helpful tool for precision mapping of the seabed. The information is useful for characterizing the seabed material properties and detecting small features not visible in thesounding data. This system employs approximately 20% overlap to ensure good quality dataand complete coverage of the seafloor. Smaller footprints are always expected for higher resolution. A narrower beamwidth results in a small sonar footprint on the seafloor. Most

MSB use a beamwidth that can vary from 0.5 degrees to 2.0 degrees. Depending upon theapplication, it is useful to combine 2 or 3 multibeam systems operating at differentfrequencies into one system. A higher frequency instrument will have better resolution andaccuracy than a lower frequency instrument. In summary the main advantage of a modernMSB is its ability to provide simultaneously a high-resolution accurate bathymetry map and a

backscatter image of the surveyed area.

6. LIMITATIONS OF ACOUSTIC BATHYMETRY

The bottom echo is generally affected by the side lobes of the beams and reverberateson all the beams. At distances longer than the bottom depth, a strong background noiseappears, due to this side lobe effect and multiple paths of these bottom echoes [29].

MSB systems are labour intensive and time consuming. It is also not possible to applythis method everywhere, due to the limitation of ship routes. Further limitations of MSB are


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due to minimum depth requirements for the vessels and possible motor restrictions on rivers.Very high frequency systems (300 kHz and above) are required for collecting swath

bathymetry in shallow water of 50 meters or less. The coverage of MSB is a function of swath

width. Most systems provide coverage of two to approximately seven times the water depth.

7. DISCUSSION

The hydrographic charts f or many of the worlds coastal areas are either out of date or nonexistent [38]. On the other hand, the use of coastal areas by commercial and recreationalconcerns is growing at a rapid pace. The coastal bathymetric data is also use for environmental hazards study. However, a huge backlog exists. Due to the budget cut, NOAAnow has only four hydrographic ships in operation (Thomas Jefferson, Rainier, Rude andFairweather). There is clearly a need for faster and cheaper approach to provide coverage of less demanding surveying areas. Survey planners are increasingly challenged to identify thetypes and locations of surveys needed and to assign the appropriate mix of data collectionsensors in order to improve survey efficiency and reduce the cost. Many survey planners arestill not fully aware to the relative merits of ALB. The following discussion will aid thedecision makers to choose the accurate type of survey methods for their survey. In general,the accuracy of any ALB system degrades somewhat from the mentioned values for cases of weak signals near the penetration limit, for situations with extremely dirty water and for depths associated with steeply sloping bottoms and small targets [19, 39, 40]. Steep bottomslopes represent a vertical and horizontal measurement problem for LIDAR as they do for MSB. Results will also vary somewhat depending on the direction of flight with respect to theslope because of the off-nadir beam angle [41]. ALB survey O&M costs depend on surveyscenarios, project location, size, horizontal density requirements, survey accuracy

requirement, re-flying, the physical shape of survey area and if it includes surf zone or highcliffs, the positioning method employed and environmental conditions on site. Many of thesesame factors also apply to water borne bathymetry survey costs. The size of the survey must

be large enough to amortize these costs. In general, large areas are cheaper to survey per unitarea than small areas. Typically acoustic bathymetry incurs k$20 to k$150 per day duringtheir survey. The O&M area cost ratio between ALB and multibeam sounders appears a factor in between 3 and 10 in favour of ALB for depths under 50 meters. Regardless of the exactnumber, it is quite clear that ALB offers a significant cost advantage in addition to its other advantages. In case of water turbidity and organic matters ALB performs better than MSB,

because of the existence of several color laser lights which result less attenuation [42]. Research can be conducted to solve the problem in the design of a laser bathymetric that

involves the precise and reliable determination of the location of the air/water interfaceconsidering a wide variety of environmental [43, 15]. The complexity of the environment andinteractions of the LIDAR beam with the environment (such as returns from fish, zoo

plankton and underwater scattering layers) made it impossible to compute all depths withhighest accuracy and reliability in real time [33]. However, for many applications in water less than 50 m deep, where the assured detection of small-objects is not as important, the ALBis a preferred option.

8. CONCLUSION

Acoustic methods are old but convenient for ocean bathymetry. In deeper water acousticis required, but is limited in shallow water. Modern ALB systems have sophisticatedalgorithm and efficient data processing packages; still there are cases where the output couldnot meet the IHO standards. Both sonar and ALB have pros and cons. Improvements in the


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systems will attract more users. The overlapping boundaries of finest performance provide theopportunity for cooperation, rather than competition, to maximize overall survey efficiencyand safety. It was experienced that, laser and sonar, have been used together with success

[11]. The Royal Australian Navy has achieved long term cost reductions to 20-30% of traditional total survey cost by using laser airborne depth sounders in combination with itssurvey vessels [8]. The combined methodology also followed in Lake Tahoe on theCalifornia- Nevada border. In short, ALB is not, and was not intended to be, a generalreplacement for MSB. It is, rather, a new tool that can be utilized, with benefits under the

proper circumstances, as an adjunct to MSB.

ACKNOWLEDGEMENT

This Research was supported by NSERC discovery grant.

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